Answer:
The build up of
toxins in the food chain is an example of biomagnification.

The toxins that become magnified are those that do not readily
break down and are usually retained in fatty tissue. An example
of such a toxin is DDT (dichlorodiphenyltrichloroethane).

If we think of insects that have been poisoned by DDT and die,
and the bodies end up in, for example, a lake, several things
will happen. The insects sink to the bottom of the lake, where
the DDT is picked up by organisms at the bottom of the food chain.
These might be algae, which will only pick up a minute quantity
of DDT, say 1 nanogram. Next, plankton may eat a thousand of
these algae, which means it now has accumulated 1 microgram of
DDT. A small fish could now eat a thousand of the plankton, which
means it has now accumulated 1 milligram. Larger fish will eat
a number of the smaller fish and in turn will increase the amount
of the DDT in their bodies. Eventually, a bird, such as an osprey,
which is higher up the food chain, will eat a number of fish
and increase the amount of toxin up to a considerable number
of grams. The effects can now be quite devastating, with the
possibility of the toxin weakening eggshells which can result
in chicks being crushed in the nest, and of course, eventually
no more ospreys.

So, the concentration at the end of the food chain can be millions
what it was initially in the environment. What started off as
relatively unharmful can become a killer due to the cumulative
effect of the toxin.